This invention relates to driving discharge lamps.
High-intensity discharge (HID) lamps, specifically pulse start metal halide discharge (MHD) lamps, place demanding requirements on the ignition pulse. The ignition pulse amplitude specified by the lamp manufacturer is typically high, though it varies between manufacturers. For example, for reliable starting, Philips Lighting specifies a pulse peak 10 greater than 3.3 kV (see FIG. 1). Apart from the peak pulse amplitude, the pulse must be sufficiently wide 12, typically 1.5-2.5 .mu.s.
In electronic ballast, the required ignition pulse is most commonly generated by charging a capacitor and then discharging it into the primary of a pulse transformer. The secondary to primary turns ratio of this transformer is typically greater than 100. A very high voltage is thus generated across the secondary. This voltage is coupled across the lamp to strike the arc. Various other components such as inductors, resistors and capacitors are used to shape the ignition pulse to meet the requirements set forth by the lamp manufacturers. Design of the ignitor circuit is one of the most critical parts in the ballast because it can affect the lamp life. The peak pulse requirements are relatively easy to meet; but the pulse-width requirements cannot be met easily. The high peak value adds stress on the ignitor transformer and requires special insulation and construction. Also, the peak current in the primary circuit can be as high as 15-20 A. U.S. Pat. No. 5,517,088 describes one implementation. The pulse peak and the pulse-width requirements add significant cost to the ignitor circuit.
As seen in FIG. 2, pulse ignition also adds minimum open-circuit voltage (Voc) requirements since the open circuit voltage determines the glow to arc transition. Most lamps are designed to operate on AC voltage/current and this makes the time 14 from application of the ignition pulse to voltage polarity reversal critical. If the polarity reversal occurs too soon after the ignition pulse, the arc may not go completely from glow to arc transition and it may extinguish. The requirement for this minimum time normally implies that the frequency of voltage applied across the lamp during starting must be sufficiently low (20-30 Hz) and must be increased (150-200 Hz) once lamp starts. This requires extra control circuitry.
In a typical pulse ignitor, the lead capacitance degrades the ignitor performance. For this reason the lead lengths are typically kept to a minimum. U.S. Pat. No. 5,517,088 describes an implementation that reduces this effect.
Starting the lamp at high-frequency (&gt;30 kHz) lowers the required peak of the ignition voltage. It is believed that applying a burst of high-frequency pulses for a period of time is equivalent to applying a wide pulse for that time. The fast transition from a peak of one polarity to the peak of opposite polarity makes the peak-to-peak voltage and not the peak voltage the effective ignition voltage. Capacitive discharge currents at high-frequencies within the arc-tube and to nearby ground planes may also play some role in the reduction of the required pulse amplitude. It has been found that the peak pulse requirements are reduced almost by a factor of two at high frequency. Experience has shown that if the peak is kept below 2.5 kV the stress on the ignitor transformer is considerably reduced. Also, the risk of corona breakdown between ballast terminals and within the ignitor transformer is significantly reduced. Corona breakdown becomes an issue when the lamp fails and the ballast continues to apply high voltage pulses to try to start it. To minimize potential problems under such a situation a shutdown circuit is required that shuts down ballast operation after a predetermined time, typically 20 minutes. Since high-frequency starting significantly reduces the peak voltage requirements it makes the system more reliable, and perhaps smaller and cheaper.
In the realm of fluorescent lamps, resonant circuits are popular for operating fluorescent lamps at high frequency. These circuits have significantly reduced the cost and size of electronic fluorescent ballasts. A resonant circuit also allows ease of starting of the lamp since high voltages can be easily generated in an unloaded series-resonant circuit.
HID lamps typically cannot be operated at high-frequency due to acoustic resonance problems. One major lamp manufacturer has a specially tuned high-frequency ballast for their HID lamps. This ballast utilizes a series-resonant circuit that starts the lamp at high-frequency. A new high-frequency technique utilizing white noise modulation is discussed in the latest work done by Laszlo Laski at Texas A&M University, "High-Frequency ballasting Techniques For High-Intensity Discharge Lamps," Ph.D. Dissertation, 1994. This approach also utilizes a series-resonant circuit in the output. This high-frequency technique is very new and there is not enough test data to assure its universal application. Thus, low frequency square wave operation remains a most popular technique for electronic HID ballasts.
As seen in FIG. 3A, a typical electronic HID ballast 30 is a three-stage power processing device. The first stage is a boost power-factor correction (PFC) stage 32. This stage insures that the current drawn by the ballast is in phase with the line voltage 33 and has low distortion. The second stage is a buck power control stage 34. This stage regulates the lamp power and limits the current in the lamp during the warm-up phase. The final stage is a full-bridge inverter 36 that takes the buck regulator's output, which is DC, and converts it to a low-frequency square wave (AC) for the lamp. In addition to these stages there is also a pulse ignitor circuit 38. Some ballasts (see FIG. 3b) combine 39 the buck stage and the output full-bridge inverter. A pulse ignitor 38 is invariably required to ignite the lamp.
U.S. Pat. No. 4,912,374 describes a high-frequency resonant ignition topology in which the power control stage and the inverter stage are combined in a half-bridge/full-bridge topology (FIGS. 5a and 5b). A disadvantage of this scheme is that since the power control (buck) stage is combined with the output inverter, in order to prevent acoustic resonance, the output inductor 20 and the capacitor 22 across the lamp must provide sufficient filtering to keep the high-frequency component of the lamp current to a minimum. Consequently, the value of the capacitor is large, in the order of 1/10 micro farad. This scheme operates the lamp at high-frequency and low frequency alternatively. When this circuit is operated at high-frequency and the lamp is off, the resonant circuit formed by the inductor and capacitor produces high voltage to ignite the lamp. Because of the large capacitance value and relatively smaller inductor value very large circulating current flows in the circuit. This large circulating current must be supported by all components of the circuit causing high stress on all parts. When the lamp is and operating in the high-frequency mode, the circuit produces high-frequency current in the lamp. During the low-frequency mode, the switching pattern is changed to one that would control the lamp power and limit the lamp current. This scheme calls for increased complexity of the control circuit and the circuit components must be selected carefully.
Another scheme, similar to the one above, is described in Japanese patent 94P01476. Here (FIG. 6) the power control and current limiting function is provided by a preceding buck converter stage. The advantage of this circuit over the ones described in U.S. Pat. No. 4,912,374 is that the value of the capacitor will be much lower and, thus, the circulating currents are not as high. A disadvantage of this scheme is that it needs four high-frequency switches 24, 26, 28, 30 and the high side switches need to be driven by more expensive drivers 21, 23 to achieve efficient drive. The lamp is again operated at high-frequency 25 and low-frequency 27 alternately.
In the above schemes during the time the lamp operates at high-frequency, the current is largely determined by the value of DC bus voltage 31, frequency of operation, and the value of the inductor. The DC bus voltage is normally fixed by other considerations. Frequency can be used to control the current to a certain extent, but since the circuit must operate close to resonance to produce high-voltage, it cannot be used as an effective control. For proper operation, sufficient current must flow through the lamp during high-frequency operation. This implies that the value of inductance must be low. To keep switching losses low, the resonant frequency must be kept low. To keep the resonant frequency from increasing because of lower inductance value, the value of the capacitor must be increased. The overall effect is that the circulating currents increase. To reach a compromise the lamp current during high-frequency operation may be lower than the desired value.